Electrochemical cells, especially high energy density cells such as those in which lithium is an active material, are subject to leakage or rupture which, in turn, can cause damage to the device which is powered by the cell or to the surrounding environment. In the case of rechargeable cells, the rise in internal temperature of the cell can result from overcharging. Undesirable temperature increases are often accompanied by a corresponding increase in internal gas pressure. This is likely to occur in the event of an external short circuit condition. It is desirable that safety devices accompany the cell without unduly increasing the cost, size or mass of the cell.
Such cells, particularly rechargeable cells utilizing lithium as an active material, are subject to leakage or rupture caused by a rise in internal temperature of the cell which often is accompanied by a corresponding increase in pressure. This is likely to be caused by abusive conditions, such as overcharging or by a short circuit condition. It is also important that these cells be hermetically sealed to prevent the egress of electrolyte solvent and the ingress of moisture from the exterior environment.
As set forth above, if such a cell is overcharged, self-heating occurs. Charging at too rapid a rate or overcharging can lead to an increase in the temperature. Overcharging the cell may occur if the charging voltage or charging current becomes too high causing the cell to rapidly overheat, thus posing a safety concern. When the temperature exceeds a certain point, which varies depending upon the chemistry and structure of the cell, an undesirable and uncontrollable thermal runaway condition begins. In addition, because of the overheating, internal pressure builds up, and electrolyte may suddenly be expelled from the cell. It is preferable to initiate controlled venting before that takes place. PTC (positive thermal expansion coefficient) devices whose resistance increases with current passage therethrough have been used in attempt to prevent excessive current throughput through a rechargeable cell. However, such devices alone are inadequate in preventing a thermal runaway condition from occurring if the cell becomes overcharged, for example, if excessive charging voltage is employed.
Conventional cell designs employ an end cap fitting which is inserted into an open ended cylindrical casing after the cell anode and cathode active material and appropriate separator material and electrolyte have been inserted into the cylindrical case. The end cap is in electrical contact with one of the anode or cathode material and the exposed portion of the end cap forms one of the cell terminals. A portion of the cell casing forms the other terminal.